Creating a Timer

In an assignment, I need to create a timer, and it's not so much that the timer doesn't work, but it doesn't display the numbers in the format I'd like. I'd like to restrict it to "ss:ff" but, it shows up as "s.fffff".

This is the code that I have so far:

using UnityEngine; using System.Collections;

public class timer : MonoBehaviour {

 public bool running = true;
 private float timeElapsed;
 public TextMesh watchMesh;
 public gateCollision TimeEnd;

 private bool startTimer = false;

 // Update is called once per frame
 void Update () {
     if (startTimer) {
         timeElapsed += Time.deltaTime;
         watchMesh.text = timeElapsed.ToString();
     }
 }
     public void StartTimer(bool start)
     {
         startTimer = start;
     }

}